The Botanical Society of Washington was founded in 1901 by professional botanists in Washington, D.C. In the beginning the Society was a place to promote discussion of botanical investigations and to provide a meeting place for other likeminded individuals. They welcome anyone with an interest in botanical topics. With meetings every month, along with field trips, they also present programs by speakers of interest to their members.
The Society publishes a Flier on its website that you can see on their main page.
Blue Vervain, Verbena hastata, is a native plant found along streams and bogs. This candelabra shaped plant attracts both bees and butterflies.
City of trees : the complete field guide to the trees of Washington, D.C. by 
Checklist of the vascular plants of Plummers Island, Maryland : contribution XXIX to the Natural history of Plummers Island, Maryland by 
Finding Wildflowers in the Washington-Baltimore Area by Highlights in the History of the Washington Botanical Society was written by John A. Stevenson in 1970. It covers the distinguished accomplishments of the Society over its long and colorful past.
According to their website, the U.S. Botanic Garden is one of many botanic gardens worldwide that actively participate in the conservation of endangered species by maintaining live specimens in their collections, studying wild plants at risk, banking seeds of rare plants, and introducing rare plants to the horticultural trade.